4.3 The Set Builtin

This builtin is so complicated that it deserves its own section.

set

set [--abefhkmnptuvxBCHP] [-o option] 
  [argument ...]

If no options or arguments are supplied, set displays the names and values of all shell variables and functions, sorted according to the current locale, in a format that may be reused as input. When options are supplied, they set or unset shell attributes. Options, if specified, have the following meanings:

-a

Mark variables and function which are modified or created for export to the environment of subsequent commands.

-b

Cause the status of terminated background jobs to be reported immediately, rather than before printing the next primary prompt.

-e

Exit immediately if a simple command (see section 3.2.1 Simple Commands) exits with a non-zero status, unless the command that fails is part of an until or while loop, part of an if statement, part of a && or || list, or if the command's return status is being inverted using !. A trap on ERR, if set, is executed before the shell exits.

-f

Disable file name generation (globbing).

-h

Locate and remember (hash) commands as they are looked up for execution. This option is enabled by default.

-k

All arguments in the form of assignment statements are placed in the environment for a command, not just those that precede the command name.

-m

Job control is enabled (see section 7 Job Control).

-n

Read commands but do not execute them; this may be used to check a script for syntax errors. This option is ignored by interactive shells.

-o option-name

Set the option corresponding to option-name:

allexport

Same as -a.

braceexpand

Same as -B.

emacs

Use an emacs-style line editing interface (see section 8 Command Line Editing).

errexit

Same as -e.

hashall

Same as -h.

histexpand

Same as -H.

history

Enable command history, as described in section 9.1 Bash History Facilities. This option is on by default in interactive shells.

ignoreeof

An interactive shell will not exit upon reading EOF.

keyword

Same as -k.

monitor

Same as -m.

noclobber

Same as -C.

noexec

Same as -n.

noglob

Same as -f.

nolog

Currently ignored.

notify

Same as -b.

nounset

Same as -u.

onecmd

Same as -t.

physical

Same as -P.

posix

Change the behavior of Bash where the default operation differs from the POSIX 1003.2 standard to match the standard (see section 6.11 Bash POSIX Mode). This is intended to make Bash behave as a strict superset of that standard.

privileged

Same as -p.

verbose

Same as -v.

vi

Use a vi-style line editing interface.

xtrace

Same as -x.

-p

Turn on privileged mode. In this mode, the $BASH_ENV and $ENV files are not processed, shell functions are not inherited from the environment, and the SHELLOPTS variable, if it appears in the environment, is ignored. If the shell is started with the effective user (group) id not equal to the real user (group) id, and the -p option is not supplied, these actions are taken and the effective user id is set to the real user id. If the -p option is supplied at startup, the effective user id is not reset. Turning this option off causes the effective user and group ids to be set to the real user and group ids.

-t

Exit after reading and executing one command.

-u

Treat unset variables as an error when performing parameter expansion. An error message will be written to the standard error, and a non-interactive shell will exit.

-v

Print shell input lines as they are read.

-x

Print a trace of simple commands and their arguments after they are expanded and before they are executed.

-B

The shell will perform brace expansion (see section 3.5.1 Brace Expansion). This option is on by default.

-C

Prevent output redirection using '>', '>&', and '<>' from overwriting existing files.

-H

Enable '!' style history substitution (see section 9.3 History Expansion). This option is on by default for interactive shells.

-P

If set, do not follow symbolic links when performing commands such as cd which change the current directory. The physical directory is used instead. By default, Bash follows the logical chain of directories when performing commands which change the current directory. For example, if '/usr/sys' is a symbolic link to '/usr/local/sys' then:

$ cd /usr/sys; echo $PWD
/usr/sys
$ cd ..; pwd
/usr

If set -P is on, then:

$ cd /usr/sys; echo $PWD
/usr/local/sys
$ cd ..; pwd
/usr/local

--

If no arguments follow this option, then the positional parameters are unset. Otherwise, the positional parameters are set to the arguments, even if some of them begin with a '-'.

-

Signal the end of options, cause all remaining arguments to be assigned to the positional parameters. The -x and -v options are turned off. If there are no arguments, the positional parameters remain unchanged.

Using '+' rather than '-' causes these options to be turned off. The options can also be used upon invocation of the shell. The current set of options may be found in $-. The remaining N arguments are positional parameters and are assigned, in order, to $1, $2, ... $N. The special parameter # is set to N. The return status is always zero unless an invalid option is supplied.